Today, trace metals are being determined at the ultra-trace level (part per billion concentration). Existing systems make accurate determinations at these levels laborious and predominantly single element processes, as typified by current commercially available atomic absorption spectroscopy (AAS) instruments. These instruments use a different light source (hollow cathode lamp (HCL)) for each element. Initial attempts at a multielement system used combinations of hollow cathode lamps and very large polychromators typically used for arc or spark source emission spectroscopy. The combined sources were inadequate, and the read out devices were bulky and cumbersome. The next generation of attempts, as exemplified by F. S. Chauang et al in Anal. Chem., 50, 525-530, (1978) and E. G. Codding et al in Anal. Chem., 52, 2133-2140 (1980), used similar combinations of HCLs with a linear photodiode array as a detector for conventional monochromators. The diode array was used to cover the largest wavelength possible (30 to 200 nanometers). The resulting instruments gave poor detection limits and were limited by the size of the array to analysis of only a few elements simultaneously. By contrast the present invention provides state of the art detection limits, and great versatility in the selection of elements to be analyzed simultaneously.
Another previous system is described in U.S. Pat. No. 4,300,833 issued 11/17/81 to Harnley et al, which employed a continuum source with mechanical wavelength scanning, a high dispersion polychromator, and photomultiplier tube detection for multielement absorption measurements. This system achieved simultaneous determination of 16 elements, excellent background correction, large calibration ranges, and state of the art detection limits for all elements with analytical wavelength above 300 nm. However, below 300 nm detection limits (signal-to-noise ratios) were worse than state of the art, because continuum sources have a reduced output in this region. This is a disadvantage of notable consequence because approximately one third of elements of interest have analytical wave lengths in this region (185 to 300 nm). It has surprisingly and unexpected been discovered, that by utilization of the present invention, all of the aforementioned advantages may be achieved along with greatly improved detection limits below 300 nanometers (nm).
Oldham et al in Anal. Instr., 16, 263-274 (1987) describe a system employing a modulated continuum source and a diode array (typically covering a 200-400 nm range) as a HPLC detector for molecular absorption. Oldham's purpose in modulating the source was to stabilize the lamp to avoid the flicker component i.e. no attempt was made to increase the intensity of the lamp.
ILC Engineering Note #152, "Use of Xenon Short ARCS as Pulsed Light Sources", ILC Technology, Sunnyvale, Calif., May, 1982, describes high energy burst mode of operation of the lamp, but it was used at very low duty cycles (1 to 2%) with the goal of achieving the maximum peak intensity.
A multielement (four channels) atomic absorption spectrometer is commercially available and is described in M. Retzik et al, "Concept and Design of a Simultaneous Multielement GF-AAS" in American Laboratory, Sept. (1987). Said spectrometer utilizes graphite furnace atomization, a bank of hollow cathode lamps, and two photo-multipliers for detectors, however it is limited to four elements at a time.
Another prior system disclosed in U.S. Pat. No. 4,049,353 to Missio employs light dispersing elements which may be utilized in the present invention. The system of Missio differs from the present invention in: (1) utilizing a plasma jet emission source as contrasted with the continuum source and electrothermal atomizer of the present invention, and; (2) utilizing an array of photomultiplier tubes in contrast with the integrating array detector means of the present invention. These distinctions permit the present invention to achieve measurements 1000 times more sensitive than the measurements made with the system of Missio.
The present invention avoids the aforementioned disadvantages of the prior art, and provides the surprising and unexpected advantages of:
using a continuum light source means (CLSM) and an integrating array detector means (IADM), said IADM dedicated to the measurement of light intensities over a short wavelength region e.g. of less than about five nm around the analytical wavelength of the element of interest; PA1 using one or more IADMs to measure light intensities over a short wavelength region around the analytical wavelengths of a plurality of elements of interest; PA1 using a burst mode of operation of the CLSM, said burst mode being exemplified by a lengthy idle period (e.g. of about 2 to 3 minutes) followed by a short time period (e.g. of about 10-15 seconds) of a series of short pulses (e.g. of about 1-5 milliseconds each) with an approximately 10-30% duty cycle, allowing the CLSM to exceed the maximum power rating by about 40-80%; PA1 using a burst mode to give enhanced average source intensities over said short time period of a series of short pulses, by increasing the effective black body temperature of the CLSM and increasing the output intensity in the far ultraviolet region (as compared to a lamp operated in the direct current mode at a normal maximum power level) by pulsing the CLSM; PA1 permitting said measured intensities over the short wavelength region to be converted to values proportional to concentration and independent of intensity as a function of time and wavelength; PA1 the high quantum efficiency of the diode array, the multiplex advantage of observing the spectral region simultaneously, a large image width, and the enhanced intensity of the burst mode, resulting in an increased intensity throughput exceeding two orders of magnitude; PA1 utilizing short length of each array, short source pulse, and high speed data acquisition to allow a high repetition rate of at least 40 times per second so that rapid, transient signals may be characterized; PA1 a combination of the foregoing advantages providing a synergistic effect whereby a multielement atomic absorption spectrometer is created with state-of-the-art detection limits for trace metals at all wavelengths; PA1 eliminating the need for a mechanical wavelength modulation device, allowing longer monitoring of each wavelength position, providing a higher quantum efficiency, and allowing better signal-to-noise ratios for trace metals with analytical lines in the far ultraviolet region, by use of an IADM. PA1 a process comprising, atomizing a sample to be analyzed; illuminating the atomized sample with a CLSM for emitting intensities from about 180 to about 800 nm, to produce a resultant light; directing the resultant light through a light dispersing means; detecting light simultaneously at more than one wavelength at a focal plane of the light dispersing means using an IADM, spanning a small wavelength region of about 4 to about 10 times the image width, for integrating with respect to time the amount of light incident thereon and for converting the incident light into amplified electrical signals proportional to the integrated intensities of the incident light; blocking the incident light from striking the detector means; during the step of blocking utilizing the detector means to convert integrated intensities into said signals; deriving from the amplified electrical signals a value proportional to concentration and independent of the intensity as a function of time and wavelength, and; repeating the steps of blocking and converting at a rate of at least 40 times per second, and; PA1 an apparatus comprising, means for atomizing a sample to be analyzed; CLSM for illuminating the atomized sample with intensities from about 180 to about 800 nm; light dispersing means; light directing means for directing light from the CLSM through an atomized sample and into the light dispersing means; IADM, located at a focal plane of the light dispersing means, and spanning a wavelength region of about 4 to about 10 times the image width, for integrating with respect to time the amount of light incident thereon and for converting the incident light into amplified electrical signals proportional to the intensities of the incident light; means for blocking the incident light from striking the detector means; control means, operably associated with the means for blocking and the IADM, for controlling the means for blocking and for signaling the IADM to readout and for receiving from the IADM readout which is amplified electrical signals proportional to the integrated intensities, while incident light is blocked from striking the detector means; deriving means, operably associated with the IADM, for deriving from the amplified electrical signals a value proportional to concentration and independent of the intensity as a function of time and wavelength, and; wherein the means for blocking and control means function to provide their respective functions at a rate of at least 40 times per second.
These and other objects and advantages of the instant invention, which will become readily apparent from the ensuing description, are accomplished either individually or cumulatively by: